Sensors, as well as energy-autonomous sensors, for detection of measurement values and other physical variables are known, for example, from EP 0918212 A1. This document discloses sensors which detect measurement values and pass on the measurement values to an evaluation device which is, in particular, central and operates on a computer-aided basis. The word “central” means that a plurality of sensors communicate with the same evaluation device. In order in this case to avoid complex, costly and visually highly disadvantageous wiring and an operating time which is limited in the case of battery and rechargeable battery operation and the need, associated with this, for the replacement and charging of the electrical power source at regular intervals, sensors, in particular temperature sensors, are proposed there, which detect measurement values and pass on the measurement values to an evaluation device which is, in particular, central and operates on a computer-aided basis, with the measurement values being passed on by means of radio transmission from a sensor device, which is connected to the sensor and has an antenna, to an evaluation device, and the transmission device being supplied with electrical power from an electrical power generator which converts the energy in the surrounding area.
In order to detect physical variables whose energy cannot sufficiently be converted to electrical energy, so that the sensor can thus be operated, such as moisture or humidity, it is necessary to convert other energy forms which are available at the location of a sensor to electrical energy, or to provide energy stores in situ, for example batteries or rechargeable batteries. In the case, for example, of the measurement of the moisture or humidity in the immediate vicinity, close to the ground, of a plant, it is not possible to assume that there is either sufficient thermal or kinematic energy for conversion to electrical energy, so that, for example, the light or solar energy must be used for conversion to electrical energy at this point.
The document U.S. Pat. No. 5,601,236 A discloses an apparatus for moistening and growth promotion of plants, in which this apparatus also has a moisture or humidity sensor, an energy store, a solar module and a control circuit, and an external timer starts a controller program which detects the amount of charge remaining in the system and/or in the energy store and determines the charge currently being supplied from the solar cells, with this being used to determine whether sufficient energy capacities are available in order to operate the various modules in the overall system. If sufficient energy is not available, the external timer is reset by a further period and the controller program causes all of the modules to return to a sleep mode until the time interval has elapsed once again and the check commences from the start.
The document GB 2320572 A discloses a plant moisture detector with a solar cell which operates an oscillator, which in turn detects the capacitive and/or resistive impedance of the ground via two separately running lines. This value is a measure of the moisture in the ground.
The document “Low Power System for Wireless Microsensors” Proceedings of International Symosium on Low Power Electronics and Design, XX, XX, 12 Aug. 1996, pages 17-21, XP001013406 by Bult, K. et al. discloses in a general form a network of wire-free sensors for applications in the transport field, production field, biomedical and environmental field and for safety and security systems. Numerous microsensors with a very low power consumption are connected to a central evaluation station via a radio link.
The document “Power Aware Wireless Microsensor Systems” ESSCIRC, September 2002, XP002288893 Florence, Italy published by Chandrakasan, A. et al. discloses a low power transmitter with a short starting time, which reduces the amount of energy required per transmitted bit of a radio link.
A system for measurement of moisture or humidity with radio transmission of the measurement values to a central evaluation device and with a solar energy power supply to the sensors is proposed in U.S. Pat. No. 4,396,149. In this case, a continuous measurement is carried out by means of the sensor, and energy is supplied continuously by means of a photovoltaic element. Photovoltaic elements of adequate size must be provided in order to provide an adequate power supply for the sensor element with radio transmission and for continuous measurement. This is not desirable and advantageous for all applications. If a sensor element is intended to provide its service in such a way that it is small, inconspicuous and reliable, in particular even at times when the light is poor, for example, at night, then neither the apparatus according to EP 0918212 A1 nor the apparatus according to U.S. Pat. No. 4,396,149 is suitable.